Various applications exist for the separation of gaseous mixtures, and in particular for the separation of nitrogen from atmospheric air to provide a source of highly concentrated oxygen. These applications include the provision of elevated concentrations of oxygen for patients requiring the same in their breathing air and for flight personnel. Additional applications relate to processes such as drying high-purity gases such as separating hydrogen from hydrocarbons.
U.S. Pat. No. 2,944,627, issued Jul. 12, 1960, to Charles Skarstrom illustrates an early apparatus and method for fractionating gases having first and second fractionating vessels packed with molecular sieve material which selectively adsorbed one or more components of the gas so as to pass through an enriched product gas. A cross-over valving assembly allowed for a flow correspondence between the vessels and with a waste gas discharge. Product gas from a vessel was channelled to a primary product outlet with a large fraction being channeled to the other vessel. This fraction flushed the adsorbed or waste gases which had been trapped by the other vessel. The cross-over valve assembly cyclically switched the connection of the vessels with the incoming gas and the waste gas discharge. This cyclic switching of the vessels provided a regular flow of the primary product gas from the primary product outlet.
U.S. Pat. No. 3,313,091, to Berlin, improved upon the Skarstrom system through the utilization of a vacuum pump to draw adsorbed or waste gases from the vessel or bed being purged. Additionally this invention utilized a more complex valving system to produce a cycle which included vessel or bed pressure equalization, repressurization product production, bed pressure equalization, dumping, and purging.
U.S. Pat. No. 4,222,750, to Gauthier et al. related to a specifically defined timing cycle in which primary product gas from the adsorbing bed was passed through the desorbing bed during the desorption cycle. The vessels were connected to a compressor during a period of adsorption and to a vacuum pump during a period of desorption.
U.S. Pat. No. 4,449,990, to Tedford Jr. improved upon these prior art patents by teaching a method and apparatus for fractionating oxygen in which a pair of molecular sieve beds were cyclically connected in a timed cycle by a first cross-over valve (i.e., a four-way valve) with a source of pressurized air and a method of depressurizing the bed. The outlet ends of the beds were further connected by a flow path referred to as a pressure equalization flow path including a pressure equalization valve ("PE" valve) for selectively opening and closing the flow path. The path included two flow conduits including a limited conduit which is always open and a regulated flow conduit which has the PE valve for variable flow rate. Further in that patent, a timing and control circuit regulated the cross-over valve such that the pressure equalization valve was open 1 percent of the cycle duration before the cross-over valve reversed positions and was closed 2 percent of the cycle duration after the cross-over valve changed positions.
Generally in the prior art as represented by these and other patents, an equalization valve is disposed between a pair of check valves at the outlet ends of a pair of sieve beds in an oxygen concentrator system. While the equalization valve was referred to by Tedford as a pressure equalization valve (i.e., a "PE" valve), in this invention we will refer to the corresponding valve as a concentration equalization valve (i.e., a "CE" valve). Ultimately the same result is achieved of allowing a purge supply of product gas to enter a used bed; however, with a pressure-based supply, the rationale for using the valve varies slightly. Specifically the equalization valve acts to dampen the oscillation of the output gas concentration into the product tank which may otherwise occur. An oxygen concentration sensor located in a bleed line from the product tank measures and provides an indication of whether or not a certain oxygen level is met. For example, normal or acceptable operation may exhibit a green light at a reading of 85 percent or above; a yellow light may be illuminated at a reading between 73 and 85 percent; and a red light illuminates at a reading below 73 percent and the device subsequently powers down. This information is merely displayed to the patient or technician. That is, the technician manually controls the equalization valve in an effort to fine-tune the oxygen supply to the patient based on the indicator lights and oxygen readings.
In accordance with the present invention, the oxygen sensor communicates with the concentration equalization valve by means of the microprocessor which utilizes a closed-loop control to provide automated operation and optimization of oxygen levels from the sieve beds to the patient. In the prior art as represented by the '990 patent, the equalization valve is set manually. This valve provides for the cyclic flow of gas from the producing bed to the evacuated bed to provide sieve bed purge and to stabilize the oxygen content of the product gas passed into the product reservoir. Specifically, the valve settings change the time that the valve is open in one direction allowing purge gas (i.e., from one bed to the second) as compared to the time that the valve is open allowing flow in the second direction.
In the present invention, a closed-loop control circuit is provided to continuously and automatically regulate the performance of the cross-over valve, of the concentration equalization valve and the compressor. An acoustic oxygen sensor located between the check valves at the output ends of the fractionation beds communicates information to the microprocessor which is programmed to evaluate the oxygen output of the sieve bed and thereby used to control operation of the production cycle (i.e., fill, purge, evacuate).
Further in accordance with this invention, the operation of the oxygen concentrator is optimized through the direct use of information regarding the oxygen output of the sieve beds. This information can be used to trigger the pressure swing adsorption cycle. This cycle can be initiated by a sensed drop in oxygen concentration with the opening of the concentration equalization valve (or alternatively with the switching of the cross-over valve). The cross-over valve can be timed to switch thereafter. The concentration equalization valve allows oxygen from the first sieve bed to mix with oxygen from the second sieve bed. The amount of time that gas is allowed to flow into the used sieve bed is determined by the concentration equalization valve adjustment. Closed-loop feedback based on acoustically derived oxygen concentration provides the optimum cycling for the pressure swing adsorption cycle (i.e., the cycle) and for the cross-over between the pressurization and depressurization means. Optimization may change as a result of compressor age, the filter condition, and line voltage. By using an acoustic oxygen sensor to measure output, the pressure swing cycle can be directly controlled using electronic control means such as an integrated or remote microprocessor programmed with the appropriate software.
U.S. Pat. No. 5,247,826, to Frola et al., relates to an acoustic gas concentration sensor used in an oxygen concentrator wherein two piezoelectric transducers are interconnected by an elongated coiled tube which provides a flow path for the gas. Periodically and alternatively, one of the transducers is energized with a single, short burst of energy or pulse to transmit a sonic wave through the gas to the other transducer. The travel time for the sonic wave is measured and used to calculate the oxygen concentration.
U.S. Pat. No. 5,627,323, to Stern, similarly relates to an ultrasonic binary gas measuring device in which a single ultrasonic wave travels between two piezoelectric transducers and the time of travel of the wave back and forth through a flow chamber is measured and used by the microprocessor to calculate the gas concentration and/or the standard flow rate for the gas.
The present invention additionally relates to an acoustic oxygen sensor which uses a pulse (or pulsed transmission) rather than a continuous sound wave in one of two manners for monitoring the oxygen content. For example, a transmitter at one end of a predetermined path sends a pulse toward a receiver disposed at the other end. A particular cell length and period of pulse time, e.g., one second, is selected. A pulse is transmitted from one end of the cell and detected at the other end. The receiver then counts the pulses and provides a signal back to the transmitter to send another pulse upon detection of the first pulse. The cumulative number of pulses over the fixed period of time provides an alternative solution to the extended path length used in the prior art. The path length in essence is substantially increased without actually increasing the cell length. Close tolerances are not required for the cell length, i.e., the distance between the transducers, so long as the cell is calibrated using a known gas at a known temperature passed through the cell.
The change in the number of pulses is proportional to the changes in the ambient conditions. Since the time between comparative pulse transmission periods is short (i.e., around a second), it is assumed that variables such as oxygen concentration and temperature are relatively static during that period. Therefore, by positioning a pair of transducers a fixed distance apart, sending a pulse from the transmitter to the receiver, or even back again, over a fixed interval of time, the difference in number of pulses counted is therefore related to the change in the oxygen concentration.
While the present oxygen sensor is not as fast in responding as are the prior art sensors (i.e., this oxygen sensor waits an entire transmission period before calculating a value), the present sensor has the advantage of providing an easier measurement, i.e., rather than measuring extremely small increments of time, the sensor counts a substantial number of pulses sent over a larger increment of time. This represents a more cost-efficient device and also provides for a more stable calculated value. Additionally, the current invention provides a temperature sensor which is exposed to the flow path but does not project into the path so as to influence the projected calculation.